Flexible circuit cables are widely used in chip to printed circuit board (PCB), chip to substrate, optical sub assembly to PCB, and PCB to PCB interconnections. They provide high density signal routing capability in a limited space and flexible manner. However, previous methods for attaching the flexible circuit cables have a variety of disadvantages.
Direct soldering, for example, possible or preferable for single flexible circuit cable attachment or multiple flexible circuit cable attachment when the flexible circuit cables are non-overlapping. However, in overlapping flexible circuit cable scenarios, the heat reflow of the solder will impact the chips or flexible circuit cables that are already attached on the substrate. For example, as shown in FIG. 1, pre-attached cable or flexible circuit cable Flex #1 will be detrimentally affected when trying to attach flexible circuit cable Flex #2 using the direct soldering method. In scenarios when sequential flexible circuit cables need to overlap with the pre-attached cable or flexible circuit cable, (Flex #1), it is nearly impossible to wet the solder between the pads of the substrate and the second flexible circuit cable, (Flex #2). The gap created by the pre-attached cable or flexible circuit cable (Flex #1) interferes with placement of the second flexible circuit cable (Flex #2). The solder experiences difficulty in flowing between the flexible circuit cable and the substrate to create electrical connectivity and a lasting reliable bond.
The Anisotropic Conductive Film (ACF), and/or Anisotropic Conductive Paste (ACP) approach is widely used in flexible circuit cable to substrate and chip to flexible circuit cable attachments for liquid crystal display manufacturing. These processes also have several limitations. For example, these processes require the electrical pads to be embossed (raised) from the surface of the flexible circuit cable and substrate so that the conductive particles in the ACF or ACP can make contact through compression to create electrical connectivity in the Z direction. The ACF process also requires high thermal temperature to cure the film to create a bond. This high temperature can impact the chips or flexible circuit cable already attached on the substrate and therefore overlapping flexible circuit cable attachment becomes difficult with the ACF and/or ACP processes if there is a gap created by a previous attached flexible circuit cable or chips. Moreover, the conductive particle filled epoxies, traditionally used in ACP, usually have a high resistance and result in a limited radio frequency (RF) bandwidth.
Flexible circuit cable attachment can also be done using conductive epoxy. The conductive epoxy can be dispensed on the pads of the flexible circuit cable or substrate, prior to placement. Several issues exist with epoxy attachment including variance/planarity condition between the flexible circuit cable and substrate, under/over volume of epoxy, proper pressure control, limited reworkability, bond strength, and higher resistance than solder. Additionally, added complexity due to overlapping flexible circuit cables makes conductive epoxy even less attractive.
In summary, there are many challenges in attaching overlapping flexible circuit cables to a substrate, interposer or other structure. The attachment of the flexible circuit cable should not impact the assembled chips on the substrate or interposer. Any proposed method needs to overcome 1) the wetting issue between the flexible circuit cable and the substrate because of the gap formed by either previous assembled flexible circuit cables or chips, or the design of the flexible circuit cable and 2) co-planarity issue caused by pre-bending the flexible circuit cable, or unique shape such as U, S or open O shaped flexible circuit cables. The proposed method has to be operable in limited spaces, account for signal RF bandwidth and achieve low signal crosstalk for high bandwidth and high density signal trace on a single flexible cable circuit. For example, flexible circuit cable attachments are used in small assembly scenarios such as an optical sub assembly (OSA) used in pluggable transceivers, (Small form-factor pluggable transceiver (SFP, SFP+, QSFP), C form-factor pluggable (CFP, CFP2), and the like).